Leading me to climate frustration

Science seems to have had a stealth theme going last week on climate change, and it included this perspective by Anna Behrensmeyer on climate change in human evolution.

The central insight is the great difficulty of finding causes among temporally correlated events:

Another challenge is deciding what constitutes a strong case for a causal link between a climate change and an evolutionary event. We can't step into a laboratory to test the impact of climate change on the human genome, but we do have the results of natural experiments--the proxy evidence for environmental changes in continental rock sequences, as well as many fossils of hominins and other organisms that were evolving on different continents during that same time period. There is a rich body of data to draw upon, but hypotheses are often structured around an assumption that "synchronous" events in the geological and paleontological record constitute evidence for cause and effect. These hypotheses, while seductive in their simple explanation of how our species came to be, do not do justice to the complexity of the climate-evolution problem (see the figure) or to the full range of evidence and scientific methodologies that now can be brought to bear on this problem.

I'd say that sums up some frustrating problems very well. There is no climate-altering event during the past seven million years small enough that some paleophile hasn't offered it up as the key factor in human evolution. But how can you prove anything? How can you even test the hypothesis of causation for most of these?

I had to read this sentence a few times, but I think it circumscribes an essential problem:

The related notion that fluctuating lake levels provided environmental stress that drove speciation does not provide a mechanism for how this could have exerted selective pressure on the immediate ancestor of Homo and resulted in the emergence of a new genus and species.

This is the problem with almost any climate-driven hypothesis. How did the change in climate cause anything to happen? Especially considering the huge bias in the sites we have to sample. Sure the Rift Valley paleoenvironment was changing during the Late Pliocene, but how central was that area to the hominid range as a whole? It's like diagnosing the causes of the American Revolution only knowing what happened in Charleston.

What we sorely lack is mechanisms that would link climate change to fitness in hominid populations. So far, there are generally two serious options: "Trees Too Far To Walk Between", and "Volcanic Winter".

The story is especially bad for the origins of Homo in the Late Pliocene. The received wisdom is that the global climate got cooler, and East Africa generally got drier. We know the robust australopithecines appeared, as did stone tool manufacture and presumably Homo. But how are these linked? It would of course help if we knew what robust australopithecines ate! If we link stone tools to meat eating (reasonably), at least we have a mechanism for dietary change in Homo. But what does a drier climate have to do with that? More antelopes?

In that paragraph, Behrensmeyer is discussing the range of climate-change hypotheses for the origin of Homo, it continues:

Other proposals instead have linked human evolution with increasing aridity and climate variability. Finally, other paleoclimatic evidence indicates drier rather than wetter climatic conditions between 2.7 and 2.5 Ma [see the figure, land record (center)], bringing into question the extent of a prolonged high lake phase throughout East Africa. Although the multibasin approach to establishing regional paleoclimate trends is commendable, the proposed causal link between a wet climate phase and the origin of Homo is not yet supported by sufficient evidence to establish its credibility.

Aaarrggh! Are the lake levels a red herring? Are the global climate figures a red herring?

The stinky fish are our only trail. What we've got is the sites we've got and the climate records as they are. Is there a good reason to think they say anything interesting about the human lineage? That depends how much of the lineage we've sampled with those sites. Yuck!

John Hawks is the Vilas-Borghesi Distinguished Achievement Professor of Anthropology at the University of Wisconsin—Madison. I work on the fossil and genetic record of human evolution (About me).

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